1. Field of the Invention
This invention relates to curable organosiloxan compositions. More particularly, this invention relates to curable organosiloxane compositions capable of cohesively bonding to substrates with which they are in contact during curing.
2. Description of the Prior Art
Many methods have been proposed for imparting adhesion to organosiloxane compositions that cure by the addition reaction of lower alkenyl radicals with silicon-bonded hydrogen atoms in the presence of a platinum catalyst. Some of these methods involve addition of various types of adhesion promoters to the compositions. The addition of a silane containing both alkenyl radicals and silicon-bonded alkoxy groups or a partial hydrolysis product of the silane is taught in U.S. Pat. No. 4,196,273, which issued to Imai and Tanaka on Apr. 1, 1980, U.S. Pat. No. 4,329,273, which issued to Hardman et al. on May 11, 1982, and in Japanese Patent Publication No. 76/28309. The use as adhesion promoters of silanes containing both silicon-bonded hydrogen atoms and silicon-bonded alkoxy groups and partial hydrolysis products of these silanes is described in Japanese Patent Publication No. 82/5836. The addition of polyorganosiloxane compositions of adhesion promoters consisting of an alkoxy-containing silicon compound, an organotitanate and an organohydrogensilicon compound is disclosed in U.S. Pat. No. 4,332,844, which issued to Hamada et al. on June 1, 1982. Curable compositions wherein the adhesion promoter is a linear or cyclic polyorganosiloxane wherein each molecule contains substituents selected from lower alkenyl, silicon-bonded hydrogen, silicon-bonded alkoxy, epoxy and ester are described in U.S. Pat. No. 3,669,072, which issued to Clark and Hayes on Oct. 17, 1972, U.S. Pat. No. 4,082,726, which issued to Mine et al. on Apr. 4, 1978, Japanese patent publication No. 80/39258, and Japanese published application (Kokai) Nos. 54/58755 and 54/58756. The use of a lower enoxysilane or a partial hydrolyzate thereof as an adhesion promoter is disclosed in U.S. Pat. No. 3,892,707, which issued to Itoh and Harada on July 1, 1975.
Each of the aforementioned prior art adhesion promoters had disadvantages associated with its use. The presence of both a lower alkenyl radical and an alkoxy group in a single silane interferes with the addition reaction and slows the cure rate. Siloxanes containing epoxy or ester groups are not completely miscible with the principal siloxane ingredient, and the composition becomes cloudy following the addition. Trimethoxysilane is an extremely toxic material. The synthesis of lower enoxysilanes is both costly and time consuming, and the silanes tend to be scattered during reduced pressure degassing of the polyorganosiloxane composition.
The present inventors examined various methods to overcome the problems associated with prior art adhesion promoters. An objective of this invention is to provide a polyorganosiloxane composition free of cloudiness wherein curing is not hindered by the presence of an adhesion promoter, adhesion is not reduced by degassing of the composition, and wherein the composition exhibits excellent adhesion to substrates with which it is in contact as it cures.